1. Field of the Invention
The present invention relates to electrical battery systems. More specifically, the present invention relates to systems for protecting against failure of electrical batteries.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.
2. Description of the Related Art
Batteries are used to provide an alternative source of power for a variety of power supply schemes. As is known in the art, the failure of a cell in a battery generally creates an "open circuit" in the system in which the battery is connected. Unfortunately, in many applications, such open circuit failure would have significant adverse consequences. In satellite applications, for example, the failure of a cell of a battery would interrupt the power supplied to the processor of the spacecraft. The contents of any volatile memory may be lost and, accordingly, if and when power is restored, the system would in effect be out of place with regard to the tasks being processed. Worse yet, the system may come up in a condition which would jeopardize the spacecraft mission, viz., false booster firing.
The design of satellite processors to circumvent this possibility is expensive and typically imposes some performance tradeoffs. Accordingly, the more common approach has been to provide some means for detecting and bypassing cell failure. Two techniques are well known in the art. One involves a network of diodes in which a diode is coupled in parallel to each cell. In this configuration, so long as the cell is operative, the diode is back biased, electrically open and substantially invisible to the surrounding circuit. When the cell fails, the diode is forward biased by the current supplied by the healthy cells and provides a bypass around the defective cell. While this approach is effective and inexpensive, the inherent resistance of the diode drains energy from the other cells and dissipates it in the form of heat. The power drain forces a requirement for additional solar panels or battery cells on the system to protect against this failure mode. The dissipation of energy in the form of heat may adversely affect the operation the battery or other sensitive elements of the system. The often used technique of heat sinking of the diode adds to the fabrication cost and weight of the satellite while doing nothing to address the power drain problem.
An alternative conventional technique involves the monitoring of the power level of each cell to detect an impending failure. Voltage levels are telemetered to a controller or operator at a ground station. The operator responds to impending failure data by sending a command to the satellite to bypass the failing cell in advance of the open circuit failure. Since the bypass is effected by a short circuit with virtually no resistance, there is no power dissipated with this technique. Unfortunately, effective operation of this system requires the presence of the human operator to provide the necessary commands in a sufficiently timely manner. The obvious disadvantages of predicating successful satellite operation on the fortuitous presence of a human operator render this alternative somewhat less than totally satisfactory.
There is therefore a need in the art for an inexpensive cell bypass system which automatically protects against cell failure without draining power from the system or otherwise adversely affecting system operation.